THE WILT SITUATION

by WILLIAM H. GILLESPIE, Adm"7aistrative Assistant to the Commissioner, West Virginia Department of Agriculture, Charleston, W. Va.

ABSTRACT. Although oak wilt has been studied for more than 30 years, there are many facets of the disease that are little under- stood. Continuing Federal-State cooperative studies are geared to predicting the overall effects of the disease on future forest- management programs, but much additional research is needed before present control programs can be expanded or discarded in a conscionable way.

AK WILT, a vascular disease caused the outermost tissue. Single spores 0by the faga- have been used to artificially initiate infec- ceam (Bretz) Hunt, is potentially tion, although the incubation period seems the most serious disease to affect oak trees to be shortened when a large number of in the 21 states where it has been found to spores are used. occur. Widespread catastrophic losses have Artificial inoculations have shown that not yet been documented, although thou- the causal agent is virulent on at least 51 sands of individual centers have species of Quercus and several close rela- been located by aerial and ground surveys tives in the genera Castanea, Lithocarpw, in patchily infected forest stands. It is not and Castmopsis. It also causes defoliation known why the disease is limited to only of the Jonathan apple (Malm) and it has part of the oak range in North America, survived for some time in sassafras. The but this phenomenon is thought to be re- lack of immunity by suggests that the lated to ecological factors that directly or range may possible expand in the future. indirectly affect the pathogen or the vec- tors. The origin of the disease remains a mys- SYMPTOMS tery. First described in in the Generally, oak wilt symptoms are dis- early 1940's, it was almost immediately tinctive and may be recognized readily found in other states in the upper Mississippi from the ground .or from low-flying air- Valley. By 1951 it had been found as far planes. east as Pennsylvania, Virginia, and West The of infected trees droop or Virginia. Estimates vary as to how long it wilt, and some turn bronze or brown in has been present. color. They then fall from the tree. Often The pathogen is a vascular parasite that the leaves fall while still green, and in enters through any wound that ruptures other instances a small green island is left at the base of each. Sometimes one or more cussed in the literature, but confirmation is branches will remain green. About 6 weeks lacking. after defoliation, the Iarger branches pro- Nitidulid beetles are prime suspects in duce sucker sprouts. Late fall much of the northern part of the range be- may show no indication of the disease cause of their intimate relationship with the other than a premature fall coloration. development of specialized fungal cushions. Some of these trees refoliate normally the borers are thought to be more logical next spring and then produce symptoms, vectors in southern areas, such as in Mis- while others die over winter without pro- souri, where the fungal cushions do not ducing visual symptoms. occur in abundance. These cushions begin Disease development often varies greatly with the development of large amounts of between individuals, and it differs markedly mycelium in the vessels. This mycelium between members of the red and white oak then migrates intracellularly to the cam- groups. Red oaks are thought to be more bium, w-here it forms "mats" between the often infected. They exhibit symptoms bark and . Endoconidia are produced over the entire tree, and the tree normally in abundance on the mats as the bark is succumbs in 1 year. The foliar symptoms slowly cracked by the growth of opposing of white oaks, in contrast, are often re- fungal cushions. This creates a cavity over stricted to localized branches, and they the mat. A characteristic odor from the mat sometimes appear only during the season attracts insects which, while feeding on the of infection. self-sterile fungus, pick up conidia, both in- In some instances white oak trees are able ternally and externally, and later transport to "bury" the fungus with overgrowths of them to other mats where conidiation en- new wood. These hidden infections are sues and fertile perithecia result. Either seldom discovered unless the tree again de- or conidia can cause infections. velops foliar symptoms, as has happened, The fungus has been shown to move after a lapse of as much as 7 years. The underground through natural grafts role of such reservoirs of infection in con- between diseased and heaIthy trees. Starting tinuation of the disease has not yet been with an initial infection, the disease, par- fully accessed. Young oak trees ticularly in shallow or sandy soils, moves may wilt and suddenly drop all of their progressively outward in a more or less leaves in a pattern reminiscent of young red concentric pattern, although on steep hill- oak, but older individuals of the same sides the disease often follows the contour. species may take somewhat longer-as is Root-graft spread is most prevalent in Mid- true for the white oak group as a whole. western States, but it does occur to some The question about whether infected red extent wherever the disease has been found. oaks ever recover is often asked, but re- is infrequent between members of search has shown that the outer vessels of the white and red oak groups, but it is infected trees are plugged with gums and known to occur. tyloses as much as 4 days before foliar symptoms develop; thus the disease is prac- CONTROL PROGRAMS tically irreversible before the first falls. Oak wilt control programs have been characterized by small-scale experiments in some states and by statewide efforts in SPREAD others. Some Eastern States conduct their Oak wilt spreads both overland and un- programs in cooperation with the USDA derground. Overland dissemination is Forest Service under auspices of the Forest thought to be primarily due to certain Pest Control Act of 1947. In the upper sap-feeding beetles belonging to the family Midwestern States, the disease was thought Nitidulidae, although Pseudopityophthoms, to be too widespread, when initially dis- a genus of oak bark borers, has also been covered, to be controlled. incriminated. The role of birds, squirrels, For example, experiments in Wisconsin and other insects has been extensively dis- indicated that the rate of progress of the disease was such that virtually all oak moved from the girdle to the soil line. This would be removed within 75 to 100 years. hastens drying of the tree as well as allow- In some states, control was attempted and ing for a rapid ingress of competing fungi abandoned. In others, control was not at- such as . Infection centers are tempted because of financial or other prob- located from low-flying airplanes in both lems. states. Nontreatment or check plots were Two exceptions are Pennsylvania and located in Maryland and Kentucky. West Virginia, where statewide control The initial study was conducted from programs have been vigorously pursued for 1958 through 1963, and the results indicated years. The control techniques used in these a -reduction of diseased trees on plots states are quite different, yet both have as treated by both methods. More importantly, their goal the elimination of the mat-like the data showed that the disease intensified fruiting structures as sources of inoculum. with time on untreated plots. As the differ- Before embarking on systematic control ent treatments and untreated checks were programs, discussion by Federal and State segregrated by state, ecological factors officials in the above-mentioned states re- could not be assessed; and this fact caused volved around whether or not the costs reservations about the validity of the study. could be justified. Also, as the potential A second study, designed to remove past losses and the chances of control were not criticisms, is now being conducted. The re- immediately known, it became imporant sults of the first few years will be available to decide whether controls based upon ad- soon. One very important aspect of this mittedly insufficient knowledge should be concinuing control appraisal is the surveil- attempted. This decision had to rest upon lance of quadrangle-sized plots instead of the danger involved if control should be the more limited areas previously used. delayed, because there was the presumed Data compiled by West Virginia workers potential of the disease for eliminating the indicate that there is very little cost differ- number one timber tree in the region. Con- ential between the Pennsylvania and West sequently it was decided to move into con- Virginia treatments if the tree location is trol as rapidly as possible, but to con- first plotted on topographic quadrangles tinually adjust or modify the control pro- and if only one trip is made to each infec- gram as new information became available. tion center. There is, however, an overall cost differential due to the use of chemicals The techniques used in the control pro- and to the greater number of trees cut in grams varied from the beginning. As Fed- the Pennsylvania method. eral funds were being used in several of the programs, a cooperative study with four states was initiated in 1958 to check on the RESEARCH NEEDED effectiveness of the differing techniques The future of all oak wilt control pro- being used and the economic implications grams hangs on research-research on the of each. This study eventually became an effectiveness of present techniques and re- appraisal of Oak Wilt Control Programs search on untried comparisons. There is in Pennsylvania and West Virginia. also a great need for research into the In Pennsylvania, all diseased oaks and ecological aspects of the disease. For ex- healthly trees of the same species group ample, why is the problem restricted to within 50 feet of diseased trees are felled, only a part of the overall range of oak in and the stumps are treated with Ammate. North America? Is this a climatic limitation The diseased trees are felled to hasten dry- or is it correlative to vector distribution? ing, which in turn lessens mat formation; There are several holes such as this in the healthy trees are removed to prevent our knowledge. For example, the causal possible spread through root grafts. fungus varies in appearance, physiological The West Virginia technique involves activity, and possibly in virulence. Thus, chopping a girdle to the heartwood at a can we develop a system of continued convenient height. The bark is then re- monitoring of the to forecast the appearance of new and more virulent strains, tors that iduence the suscept-pathogen- or strains adaptable to different environ- vector relationships? Do we really under- mental conditions, which could conceivably stand the disease as it exists in nature? extend the present geographic range? There are dozens of similar basic ques- It is doubtful that the fungus is limited to tions that need to be asked and answered distribution by a single vector, or even to before we can assess the me potential of two or three; but present knowledge on oak wilt. Will the fact that the require- this matter is inconclusive. ments for successful transmission are very The roIe that antibiotics and systemic exacting and that the requirements may will ultimately play in the treat- vary from region to region work to the ment of infected trees in residential or other detriment of control activities? Perhaps dif- high-value properties needs to be more ferent control measures will need to be adequately evaluated. Will soil sterilants be developed that will consider ecological developed that will inhibit root-graft trans- peculiarities. mission in the Appalachians as methyl Perhaps we will never know the answers bromide apparently does in the sandy soils to all the questions. Perhaps the disease will of Wisconsin? silently disappear just as it arrived. Or per- What are the respective roles of spring haps it will intensify and devastate the re- and fall mats? Is fall mat production im- source before we can solve the riddle or portant? If so, does the infection occur so develop the necessary control programs. It that the fungus overwinters in the host? Or is ironic, but the overall pace of research do the spores overwinter in the soil, in or effort has slowed in recent years. Can it be on insects, etc.? accelerated? Has it slowed because the easier answers have been found? Or is it QUESTIONS, QUESTIONS because we have tired of the problem and Natural regeneration in disease centers have not yet been sufficiently remotivated? does not seem to be a problem in the Ap- Could it be that many of the past problems palachians, but perhaps not enough time has have been solved in the laboratory, while elapsed to show certainly. Will this situa- most of the current questions will be an- tion change when infection centers expand swerable only in the field? and unite so as to remove oak from acres The evaluation of oak wilt intensities in contrast to the present small centers? and the location of diseased trees through What is happening on large infection cen- use of aerial surveying techniques is a field ters in the Midwest? Will the disease ulti- of recent interest. Much can be said for mately cause a degeneration of stand pro- color and for infrared photography, but ductivity throughout its range, even though these are as yet strictly experimental and there may not be any discernable influence cannot be used on routine programs. on the establishment, survival, and develop- One problem with aerial techniques, both ment of individual oaks? in surveying for oak wilt and for beetle- Will the mere presence of the disease damaged pines, is that more or less over- continue to inhibit the exportation of oak lapping contour flying is necessary. It has logs to foreign countries? Will it be pos- been shown that it is virtually impossible, at sible to exploit the fumigation treatments altitudes of 500 feet above the terrain and worked out by the USDA Forest Service at speeds of 90 miles per hour, to locate as a technique to obviate present quarantine single trees or even clumps of two or three restrictions? infected trees more than L/4 mile distant. Are there unrecognized inoculum sources? Therefore, surve s flown on 1 mile grids, If so, what are they, and how is the fungus or flown at 500 Yeet above the highest ter- transferred? rain, or flown at speeds in excess of 100 What factors govern the establishment of miles per hour, are suspect. Even if flown the disease in an otherwise healthy tree? the exact same way every time, they will Do we understand the overall ecologic fac- not give similar results, especially if new or untrained personnel are used. In addition, total effect on forest management in time? grid flights never give 100-percent cover- Will we be able to recommend harvesting age, and they should not be reported as practices that will slow the disease? Will such. we be able to develop a statistical model to predict future intensities instead of rely- WHAT NEXT? ing upon deterministic calculations? Oak is such a valuable resource that In summay*the oak situation is every possible must be expended to now vimall~ static; and program prevent is loss if the loss indeed seems im are hadly in need of minent. The surveys and resultant cont~gm It is anticipated that within the next 2 progmms may have to be extended in- years the cooperative Federal-State analysis definitely. We jua do not know at present. will provide guidance on how to proceed We hope some sort of biological convol with present programs. will finally be developed on the basis of But, if the analysis indicates that control complete ecological data. Only then will efforts should cease, then the true problem those entrusted with the management of will rapidly emerge. Will we be able to our forest resources for future generations develop information on how to predict the be able to relax.

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